If our understanding of the universe is correct, there will come a day when every star in the cosmos has winked out, either falling apart or becoming a black hole. In such a dark time, trillions of years from now, it’s hard to imagine any intelligent life still clinging to this mortal coil. However, scientists believe that, even in this extreme environment, there is a way for a civilization to thrive for millions of years.
Of course, it would be a life lived in eternal night. The sky above would be cold and black, starless and trackless. The society would likely live on a superstructure constructed around a black hole. Why? Because it would be the last consistent way to get energy.
Black holes are the terminal point for information in the cosmos. That which falls into a black hole simply does not escape. Beyond the event horizon, even light is not fast enough to escape the gravitational pull of a black hole. What fate awaits that which falls beyond this point is impossible to know for certain, as it is an informational dead-end.
However, that doesn’t mean black holes are useless to a sufficiently advanced civilization. For one thing, they make great places to chuck your waste! However, in a more practical sense, black holes can be used to power an entire civilization, essentially indefinitely, because they are spinning.
When a black hole is formed, something very massive (a big star) becomes very tiny (as small as something can physically become) very quickly. Since stars are rotating, and their momentum is conserved thanks to inertia, the black hole, which is sub-atomically small, is still spinning.
Some black holes are spinning at millions of times per second. Here’s the thing, though: at the center of every black hole is an infinitely small point called a singularity. Since a one-dimensional point can’t spin, though, spinning black holes have what scientists have dubbed “ringularities” at their cores. A ringularity is a ring with no thickness and no surface, if you can picture such a thing.
The black hole is so powerful and spinning so fast that it warps spacetime around it. Even outside of the event horizon of a spinning black hole is a region called an ergosphere, a region where space and time are badly distorted and malleable. One would need to be moving faster than the speed of light just to stand still within the ergosphere.
This is where a black hole civilization could find itself a nearly infinite source of energy. If a civilization crafted a superstructure outside the ergosphere of a spinning black hole, it could fire something into the ergosphere for the purpose of stealing the rotational energy. For instance, they could wrap the entire structure in mirrors that completely envelop the black hole, then fire electromagnetic waves at the black hole.
The waves hit the black hole at lightspeed. The waves would bounce between the black hole and the mirrors, speeding up each time, yielding astronomical amounts of energy. All one would need to do is open a slight window in the mirror to harvest billions of megawatts of energy. This could keep a civilization’s lights on for millions of years into the far future, long after the last stars have winked out of existence.